(1). Field of the Invention
This invention relates, in general, to silk-screen printing apparatus. More particularly, the invention relates to a silk-screen print head for use in a multicolor halftone silk-screen printing process, such as the four-color silk-screen printing of the surface of a compact disc.
(2). Description of the Prior Art
Silk-screen printing, in general, involves the use of a screen, i.e., a woven mesh fabric having a plain weave, stretched over a frame and the design or text to be printed is provided on the screen in outline form, in the nature of a stencil. The design to be printed is provided in the silk-screen, in general, by coating the screen with a photosensitive emulsion, exposing the emulsion to a source of light to obtain the desired image and then washing the unexposed areas to leave the screen with the image to be printed. The design or text is reproduced on a desired object, e.g., the surface of a compact disc, by having a squeegee force color, i.e., ink, through the mesh of the exposed areas of the screen. Thus, the image or text printed on the surface of an object comprises a plurality of closely spaced dots of color.
In general, the silk-screen frame and squeegee are mounted to the print head independently of one another. The screen frame is provided with means for movement of the frame in an x-y-z direction, relative to the object to be printed, e.g., the top surface of a compact disc located in the horizontally disposed well of a disc fixture. The screen frame is of a square or rectangular shape defined by parallel side edges and parallel, opposed inner and outer end edges. The squeegee is mounted so as to move linearly relative to the inner and outer end edges of the screen frame. Where the screen frame is moved only in an x-y direction, the squeegee, in general, moves across the woven fabric screen located in the screen frame in a manner such that it's movement is square with respect to the square mesh defined by the screen. Nevertheless, if the screen frame has needed to be rotated relative to the surface to be printed and the image to be printed thereon, the squeegee will then travel across diamond-shaped mesh rather than square-shaped mesh in the woven screen. This leads to a distortion in the dots of color being printed. As a result, a moire' interference pattern occurs in the printed image. Moire', in general, manifests itself in an unsuitable shimmering pattern or wave-like appearance.
Another problem resulting from the rotation of the screen frame to register the image to be printed, relative to the surface to be printed, is that such rotation can be inhibited by the size of the screen frame. This results from the fact that the travel of the squeegee, in silk-screen printing, is fixed, i.e., the squeegee travels in a linear direction between the sides of the screen frame, while the travel of the screen frame is not. Thus, the desired rotation of the screen frame may necessitate the use of a larger size screen frame than desired so that the squeegee can fit within the side edges of the frame. Going to a larger size screen, however, can result in having to provide everything in the silk-screen printing apparatus of a somewhat larger size. This is generally undesirable, as it is likely to increase the cost of the apparatus. Moreover, it usually is most preferred to keep such equipment in as compact an area as possible, particularly if space is limited.
Halftone printing, contrary to full color printing, involves a shading or gradation of color. In such printing, the gradation of the tone of color is obtained by a system of closely spaced dots of color arranged in parallel lines, i.e., rows and columns of dots of color. For example, in the four color printing of the surface of a compact disc, this involves the separate printing of a line of dots of cyan, black, magenta, and yellow of full color, in turn. The dots of different color being printed need be provided in proper linear registration in respect to one another to provide not only the desired gradation of color but also to prevent moire'. Thus, the lines of dots of color are intended to be provided on the surface of the object being at predetermined angles, e.g., the angles of color separation, so that the lines of dots do not cross one another.
Heretofore, the manner of eliminating, or at least reducing or localizing, moire' in the silk-screen printing of halftones has been most difficult. One method suggested heretofore has been to mount the printing head so that it can be moved in an x-y-z fashion, relative to the object to be printed, e.g., the top surface of a compact disc. Thus, with rotational movement of the print head, the squeegee can still be made to move across the mesh of the screen in a square manner, i.e., the squeegee moves across squares rather than diamonds, and the lines of dots of color are printed at the desired angles of color separation. This procedure, however, has proved less than satisfactory. Such a manner of printing requires that, for example, a compact disc to be silk-screen printed be provided on a puck. The disc/puck combination is then loaded by hand at each print head for each different color to be printed. The print heads, in this case, are previously rotated to the desired angle relative to the surface to be printed and the color to be applied at that particular printing station. This manner of printing, as will be readily appreciated by those skilled in the art, does not readily lend itself to automation. This is not only time consuming but leads to an inefficient and costly manner of operation. Moreover, where the print head is itself mounted to be rotatable this tends to limit the number of print heads that can be provided in any particular silk-screen printing apparatus. This is due to the size of a conventional print head and the area that must be provided for the print heads to be rotatable. The more print heads provided, the larger the area that would be required to accommodate the rotatable print heads, the support member for the print heads and the associated transport member.
Another manner of eliminating, or at least minimizing or localizing moire' has been to change the angle of the mesh in the woven fabric screen relative to the screen frame, i.e., providing the mesh of the screens in the frames at the angles of color separation. Thus, when the screen frame is mounted to the print head, the mesh of the screen will be properly oriented so that the angles of color separation are not only taken into consideration but the squeegee will move across the mesh of the screen in a square manner. Although this manner of eliminating moire' in halftone printing is, in general, satisfactory, the providing of such screens is not totally satisfactory. Due to the limitations of conventional print heads the mesh in the screen cannot exceed a 15.degree. rotation in either direction. Where the screen frame is rotated to a greater extent than 15.degree., the squeegee is again printing across diamond-shaped mesh rather than square-shaped mesh. In such a case, the squeegee may not, at least in some instances even fit within the screen frame, as earlier disclosed. Furthermore, this manner of at least minimizing or localizing moire' is a somewhat costly proposition.
In the conventional manner of providing silk-screens, in general, a square cut of a woven fabric is stretched to the extent desired. Screen frames are then placed under the stretched fabric in side-by-side manner, after which the stretched fabric is adhesively secured to the flames. The screens are then provided by cutting around the outside edges of the frames. In such a process, there is little, if any wastage of fabric, as the frames are provided in close association with one another. Nevertheless, where the mesh of the woven screens is to be provided in the frames at predetermined angles of color separation, the screen frames must necessarily be spaced apart from one another. This can result in considerable wastage of fabric from which the screens are made, leading to increased cost for a set of screen to be used in halftone printing.
Another means of possibly eliminating moire' in any particular silk-screen printing operation is to change to a fabric with a different mesh count. Nevertheless, this is not a desirable solution. The manufacture of a set of silk-screens for halftone printing is a somewhat costly and labor intensive operation. Where moire' occurs, the making of a new set of screens with different mesh woven fabric merely compounds this expense to a printer.
Thus, it is highly desired to provide silk-screen printing apparatus whereby moire' can be eliminated, or at least localized or minimized, in a more efficient and less costly manner.